Detection and identification or at least classification of unknown substances has long been of great interest and has taken on even greater significance in recent years. Among advanced methodologies that hold a promise for precision detection and identification are various forms of spectroscopy, especially those that employ Raman scattering. Spectroscopy may be used to analyze, characterize and even identify a substance or material using one or both of an absorption spectrum and an emission spectrum that results when the material is illuminated by a form of electromagnetic radiation (for instance, visible light). The absorption and emission spectra produced by illuminating the material determine a spectral ‘fingerprint’ of the material. In general, the spectral fingerprint is characteristic of the particular material or its constituent elements facilitating identification of the material. Among the most powerful of optical emission spectroscopy techniques are those based on Raman scattering.
Raman scattering optical spectroscopy employs an emission spectrum or spectral components thereof produced by inelastic scattering of photons by an internal structure of the material being illuminated. These spectral components contained in a response signal (for instance, a Raman signal) may facilitate determination of the material characteristics of an analyte species including identification of the analyte.
Unfortunately, the signal produced by Raman scattering is extremely weak in many instances compared to elastic or Rayleigh scattering from an analyte species. The Raman signal level or strength may be significantly enhanced by using a Raman-active material (for instance, Raman-active surface), however. For instance, the Raman scattered light generated by a compound (or ion) adsorbed on or within a few nanometers of a structured metal surface can be 103-1012 times greater than the Raman scattered light generated by the same compound in solution or in the gas phase. This process of analyzing a compound is called surface-enhanced Raman spectroscopy (“SERS”). In recent years, SERS has emerged as a routine and powerful tool for investigating molecular structures and characterizing interfacial and thin-film systems, and even enables single-molecule detection.
Most SERS systems only enhance the electro-magnetic field at certain hot spots. While this can be very desirable, in many cases, the analytes are spread evenly on a SERS substrate, such as by simple adsorption. However, only a small fraction of the analytes actually populates the hot spots. In addition, conventional SERS systems employ periodic arrays of the Raman-active materials, which may cause a grating effect, such that certain angles of light propagation often yield optimal signal emission performance, while other angles often yield poor signal emission performance.